fix jsonutils with generic sandwiches, don't use strformat (#24560)

fixes #24559

The strformat macros have the problem that they don't capture symbols,
so don't use them in the generic `fromJson` proc here. Also `fromJson`
refers to `jsonTo` before it is declared which doesn't capture it, so
it's now forward declared.

tests/stdlib/mjsonutilssandwich.nim

@@ -0,0 +1,11 @@
+import std/[json, jsonutils]
+
+type
+  Kind* = enum kind1
+  Foo* = ref object
+    bleh: string
+    case kind*: Kind  # Remove these lines and everything works 🤡
+    of kind1: discard # Remove these lines and everything works 🤡
+
+proc unserialize*[T](s: string) =
+  discard jsonTo(parseJson(s), T)

tests/stdlib/tjsonutilssandwich.nim

@@ -0,0 +1,10 @@
+discard """
+  action: compile
+"""
+
+# issue #24559
+
+import mjsonutilssandwich
+# import std/[json, jsonutils] # Add this line and everything works 🤡
+
+unserialize[Foo]("{}")